1. Technical Field
This invention relates to semiconductor packages having a very thin vertical body thickness. More particularly, the present invention relates to a thin semiconductor package in which each inner lead is mechanically interconnected by an annular strip of resin attached to the plurality inner leads. This arrangement provides firm support to the inner leads of a Tape Automated Bonding (TAB) tape carrier and to a semiconductor chip joined to it. This arrangement also prevents tilting or shifting of the semiconductor chip during transfer molding processes.
2. Description of the Prior Art
Presently, as electronic devices, such as VCRs, cameras, liquid crystal displays, and memory cards, follow industry trends to smaller size and higher density, new technologies are required to produce semiconductor packages which are lighter, thinner, shorter, and smaller. Such technologies have already, or will soon, produce semiconductor packages having a thickness of only 0.5 mm.
Thin package technologies can be split into two groups: one group using wire bonding to a lead frame, and the other group using Tape Automated Bonding (TAB). In TAB technology, the semiconductor chip is encapsulated by a coating process, or by potting in a molding resin compound. Mass production of TAB devices generally requires screen printing or transfer molding. Transfer molding is an automated version of compression molding in which a plastic molding compound is forced from a pot into a hot mold cavity. Transfer molding of a thin semiconductor package requires particular care in the selection of the molding compound, or in the determination of molding compound characteristics, since the stress placed on the molding compound by ever thinner devices is extreme. Typically, the requirements of a molding compound suitable for use in the thin semiconductor packages are a low viscosity, a high flexural strength, a low thermal shrinkage, and an optimum flow of the mold resin, and the like. These conditions are more strict in the case of transfer molded TAB package.
FIGS. 1 through 4 are cross-sectional views of conventional, thin semiconductor packages 7, 8, 9 and 10. All of these packages utilize TAB technology in which a thin copper film is processed through photolithography to form a plurality of inner leads 14 and outer leads 16. A semiconductor chip 11 is bonded to the inner leads 14 via solder bumps 13. Electrical signals from an external device are delivered to chip 11 or vice verse, through a plurality of electrode pads 12 formed on chip 11.
In the conventional semiconductor packages shown in FIGS. 1 through 4, a support film 15 having a shape or structure as shown in the corresponding Figures is attached to inner leads 14, prior to the (ILB) process in which inner leads 14 are bonded to the electrode pads 12 using thermo-compression. Support film 15 functions as a tape or connecting strip for inner leads 14 during the ILB process. Encapsulation of chip 11 and leads 14 is made with a molding resin compound 17.
The conventional semiconductor package 9 in FIG. 3 is somewhat different from the other conventional packages. Package 9 is an ultra thin package in which the upper and side surfaces of semiconductor chip 11 are encapsulated in the molding resin, but the bottom surface of chip 11 remains exposed outside the molding resin.
Transfer molding generally starts after the support film 15 has been attached to inner leads 14. As shown in FIGS. 1-4, within TAB formed devices, semiconductor chip 11 is supported by inner leads 14 and through solder bumps 13 and electrode pads 12. Although support of the semiconductor chip 11 is assisted by support film 15 connected to inner leads 14, it is often not sturdy enough to withstand the enormous pressure created with the hot, liquid molding compound is transferred into the cavities of the mold.
Furthermore, when the molding compound is filled from the bottom of the cavity, the semiconductor chip 11 may be tilted to one side, if it is not exactly centered in the mold relative to the stress or pressure of the injected molding compound. The tilt or shift of the chip 11, in the worst-case scenario, creates an outward protrusion of the chip 11 from the molding compound, and thus causes the following problems.
First, in the subsequently performed reliability test, such as the Pressure Cooker Test or the Temperature Cycle Test, corrosion may occur, or deformation of the metal wiring in the protruding semiconductor chip may occur due to exposure to high pressure and temperature. These problems result in a critical failure of the chip.
Second, exposure of the leads of the TAB device may cause current leakage or a short circuit.
Third, the thickness of the molding resin on one side becomes larger than that on the other side of the semiconductor chip when the chip tilts. This difference in the thickness of the molding resin may be one of the major causes of the package cracks.